Aromaticity in heterocyclic systems III. The structure and proton magnetic resonance spectra of certain imidazo[1,2‐a]pyridines

A number of improved laboratory procedures for the synthesis of derivatives ofimidazo‐[1,2‐alpyridines are reported. These methods have been employed for the preparation of 5‐chloroimidazo[1,2‐alpyridine (IX), 5‐aminoimidazo[1,2‐a]pyridine (IV), 2‐methyl and 2,3‐dimethyl‐5‐aminoimidazo[1,2‐a]pyridine. The proton magnetic resonance spectra of imidazo[1,2‐a]pyridine (I) have been studied and the bands assigned by a comparison of spectral patterns of simple derivatives. The proton magnetic resonance spectra were then utilized to assign predominant tautomeric structures to imidazo[1, 2‐a]‐2‐pyridone (IIIb), imidazo[1,2‐a]‐(1H)‐5‐pyridone (VIIb) and 5‐aminoimidazo‐[1, 2‐alpyridine (IV). The synthesis of imidazo[1,2‐a]‐(1H)‐5‐pyridone (VII) was accomplished from 5‐chloro‐imidazo[1, 2‐alpyridine (IX) via 5‐benzyloximidazo[1,2‐a]pyridine (X) which was debeneylated catalytically to yield VII. Bromoacetaldehyde and 2‐amino‐4‐chloropyridine gave 7‐chloroimidazo[1,2‐a]pyridine.